One technique for packaging articles, particularly fragile articles, comprises packaging them in some sort of polymeric foam composition. In one particular technique, the foam is generated in place during the process of packaging the articles. For example, when certain chemicals are mixed together they form polymeric products while at the same time generating gases such as carbon dioxide and water vapor. If those chemicals are selected so that they harden following the generation of the carbon dioxide and water vapor, they can be used to form hardened polymer foams in which the mechanical foaming action is caused by the gaseous carbon dioxide and water vapor leaving the mixture.
In particular techniques, synthetic foams are formed from liquid organic resins and polyisocyanates in a mixing chamber. The mixture is preferably dispensed into a receptacle, such as a package or a foam in place bag (see e.g. U.S. Pat. Nos. 4,674,268, 4,800,708 and 4,854,109), where it reacts to form a polyurethane foam.
A particular problem associated with polyurethane foams is that once mixed, the organic resin and polyisocyanate generally react relatively rapidly so that their foam product tends to accumulate in all openings through which they pass, including the openings through which they pass before mixing.
Furthermore, some of the more useful polymers that form foamable compositions are somewhat adhesive. As a result, the foamable composition, which is often dispensed as a somewhat viscous liquid, tends to adhere to objects that it strikes and then harden in place. As stated above, this causes a particular problem in the injection nozzles or cartridges in which the foam is mixed and from which they are injected into a receptacle.
Injection systems for such foamable compositions and their operation are thoroughly described in U.S. Pat. Nos. 4,568,003 and 4,898,327, both of which are commonly assigned to the assignee of the present invention and incorporated entirely herein by reference. As set forth in both of these patents, in a typical dispensing cartridge, the mixing chamber for the foam precursors is a cylindrical core having a bore that extends longitudinally therethrough. The core is typically formed from a fluorinated hydrocarbon polymer such as polytetrafluoroethylene ("PTFE" or "TFE"), fluorinated ethylene propylene ("FEP") or perfluoroalkoxy ("PFA"). Polymers of this type are widely available from several companies, and one of the most familiar designations for such materials is "Teflon", the trademark used by DuPont for such materials. For the sake of convenience and familiarity, such materials will be referred to herein as "Teflon", although it will be understood that the materials available from companies other than DuPont can also be used if otherwise appropriate.
A plurality of openings (usually two) are arranged in the core in communication with the bore for supplying the organic resin and polyisocyanate to the bore, which acts as a mixing chamber. A valving rod is positioned to slide in a close tolerance, "interference", fit within the bore to control the flow of organic resin and polyisocyanate from the openings into the bore and the subsequent discharge of the foam from the cartridge.
As set forth in the background portion of the '003 patent, during use the interference fit between the valving rod and the bore of the mixing chamber tends to distort the Teflon material and the openings therein. This process is referred to as "cold flow" or "creep", and is generally characteristic of Teflon and of all of the related polymers The cold flow distortion of the Teflon causes several problems, including the loss of the fit between the bore and the valving rod as well as the fit between the openings through which the separate precursors enter the bore for mixing and then dispensing. Stated somewhat differently, the Teflon core is fitted in the cartridge under a certain degree of stress in order to help prevent leaks in a manner in which a gasket is fitted under stress for the same purpose. This stress, however, also encourages the Teflon to creep into any gaps or other openings that may be adjacent to it.
The '003 and '327 patents take two different approaches to solving the foam buildup problem. The '003 patent discloses a injection system in which the valving rod and mixing chamber of a portion of the injection apparatus is detachable from the remainder of the injection system so that it can be easily replaced once the foam buildup occurs or the cold flow otherwise distorts the cartridge. By making the entire cartridge easily replaceable, the '003 patent offers a useful improvement to devices and methods that preceded it. Also, the '003 patent suggests that fitting the Teflon core tightly into the shell of the cartridge will likewise help prevent cold flow.
The '327 patent takes a somewhat different approach and discloses an injection cartridge which includes a sintered porous metal insert near its exit tip. When a solvent and compressed air are forced together through the sintered portion, they generate an effervescent cleaning action at the tip of the cartridge that helps keep foam from building up. This also represented an improvement over the '003 patent which used the simple wetting of a solvent from a reservoir through the cartridge between injection cycles in an attempt to clean it.
Neither of these solutions, however, offer any improvement or solution to the cold flow problems mentioned earlier, and particularly fail to deal with the problems associated with the entry of the separate precursors into the mixing chamber and the associated fittings and hardware.
In this regard, in typical injection cartridges the separate foam precursors enter the bore through separate entry ports. Polyurethane foam tends to build up at the area at which the precursor exits the port and enters the mixing chamber. Such buildups cause spraying in the output stream, and dispensing of the mixture in an improper ratio.
In turn, foam buildups at these locations appear to be the result of two more fundamental factors. The first is crossover; i.e. unintended mixing that is caused by chemical leakage around the valving rod. It appears that this leakage is in turn caused by a poor seal between the valving rod and the mixing chamber along the path between the two ports.
A second reason is that excessive gaps exists around the port exit area. These allow polyurethane to accumulate near the exit area and gradually occlude the port.
In particular, conventional port fittings have square (i.e., flat) faces while the valving rod and bore are cylindrical. As a result, a gap always exists between the face of the port and the valving rod. This gap allows urethane to accumulate in a large percentage of such ports. Very often the polyurethane in this gap will grow into the port exit itself, causing the spraying or improper ratio problems referred to earlier.
Such gaps also give the Teflon mixing chamber a place to creep and thereby lose some of its sealing pressure, thereby aggravating the crossover problem.
Additionally, the gap at the entry side of the port can be aggravated because the a flat face typically contacts a curved surface of the cartridge housing. As a result, the port and the cartridge housing make point contact along two edge corners. In turn, high compressive loads and subsequent deformations at the contact points permit the port to move away from the valving rod at its other end. This action increases the critical gap and allows even more polyurethane to accumulate at the port exit area.
The geometry of the flat port against the round cartridge housing also creates small gaps into which the Teflon of the core can creep. This likewise probably contributes to seal failure and eventual crossover.